1. Field of the Invention
This invention generally relates to a method and an apparatus for detecting a full load state of a carrier pallet carrying a pile of machined workpieces in a sheet metal machining line for producing desired products by cutting, drilling, bending and/or otherwise machining flat workpieces according to a machining schedule prepared on the basis of a given production plan and, more particularly, it relates to a method and an apparatus for detecting a full load state of a carrier pallet carrying a pile of machined workpieces with a predetermined accuracy regardless of the size of each workpiece on the carrier pallet.
This invention also relates to a sheet metal machining line for producing desired products and/or semiproducts by cutting, drilling, bending and/or otherwise machining flat workpieces according to a machining schedule prepared on the basis of a given production plan and, more particularly, it relates to a method and an apparatus for controlling a sheet metal machining line in such a way that the time required for an entire machining cycle is minimized to maximize the efficiency of the line by analyzing the machining schedule, estimating the possibility of occurrence of a full load state as a function of the heights and the weights of the workpieces to be carried by the carrier pallet, arranging a carrier pallet in a stand-by state for the next loading cycle on a just-in-time basis by referring to the full load state estimate to thereby minimize the time required for procuring a carrier pallet.
This invention also relates to a workpiece identifying apparatus adapted to automatically identifying workpieces to be machined in terms of material, size and number without requiring human aid.
2. Prior Art
Automated power saving manufacturing systems such as flexible manufacturing system (FMS) have been known and in popular use in sheet metal machining lines for cutting, drilling, bending and otherwise treating sheet metals into intended products.
An apparatus for controlling a sheet metal machining line utilizing an FMS system will be summarily described.
Firstly, a sheet metal machining line typically comprises an NC machining center including one or more punch presses adapted to selectively use a plurality of metal molds and tools, a CNC control apparatus for controlling the operation of the NC machining center according to a machining program, a workpiece holding/releasing unit for holding one or more than one workpieces to be machined in position relative to the NC and releasing them for delivery, an automatic warehouse for sorting a large number of workpieces into groups of same materials and sizes and storing them for future machining, peripheral equipment including one or more than one cranes and other devices for feeding workpieces to the workpiece holding/releasing unit from the automatic warehouse and a line control board connected to the NC machining center, the CNC control apparatus and the peripheral equipment for controlling the sequence of operations of the sheet metal machining line.
The sheet metal machining line is additionally connected to a cell controller for transferring machining programs to the CNC control apparatus and also transferring ladder-sequence programs to the line control board in order to control the entire operation of the sheet metal machining line in a coordinated manner.
The cell controller is connected to a hard disc memory device having a predetermined capacity typically for storing machining schedules prepared according to a given production plan describing the operating procedures of the NC machining center and the peripheral equipment for sequentially producing products according to the production plan.
The cell controller sequentially issues commands for operation according to the machining schedule and the issued operational commands are then sent to the CNC control apparatus and the line control board.
Upon receiving the commands, the sheet metal machining line sequentially carries out the specified machining operations including cutting, drilling and bending on the delivered flat workpieces.
A machining schedule typically comprises more than one unit machining schedules designed for each specific product item and arranged in the order of the machining operations to be carried out.
The unit machining schedule describes data items such as the identification numbers of the machining programs involved, the use or non-use of such identification numbers, the number of workpieces to be machined the presence or absence of the predetermined procedures for exchanging metal molds and completion codes for indicating the completion of the operations specified in the unit machining schedule.
The cell controller causes the predetermined machining operations to be carried out in the order defined by the machining schedule by referring to the machining schedule whenever necessary so that products and/or semiproducts may be produced according to the machining schedule.
With a sheet metal machining line as described above, the workpieces machined by the NC machining center of the line are sequentially loaded on carrier pallets arranged in a loading site, which operates as part of the line.
Thus, the number of workpieces loaded on carrier pallets increases with time, although the capacity of each carrier pallet is limited in terms of either the height or the weight of the load it carries as a function of the height and strength of the shelf of the automatic warehouse that receives the pallet and the strength of the crane car as well as other factors.
Thus, the workpiece loading site is typically provided with an apparatus for detecting a full load state for each pallet loaded with workpieces there.
A known apparatus for detecting a pallet full load state will be described by referring to FIG. 1 of the accompanying drawings, illustrating a loading station.
Referring to FIG. 1, the loading station 201 has underneath it, a lifter 203 for supporting a carrier pallet 206 carrying workpieces 207 thereon with a skid 205 interposed therebetween.
The lifter 203 is provided with a support mechanism (not shown) such as a hydraulic cylinder and designed to support from under the carrier pallet 206 that carries workpieces 207, slightly and correspondingly moving downward as a function of the total weight of the workpieces 207 loaded on the carrier pallet 206.
A pair of top sensors 209a, 209b are arranged at opposite positions in an upper area of the inner wall of the loading station.
With the known apparatus for detecting a pallet full load state having a configuration as described above, the top of the workpieces 207 loaded on a carrier pallet 206 is detected by the top sensor when the total weight or the overall height of the workpieces 207 on the carrier pallet 206 exceeds a predetermined corresponding limit.
More specifically, if the carrier pallet 206 carries workpieces 207 whose total weight exceeds a predetermined limit, the overall height of the workpieces 207 consequently exceeds a corresponding limit so that the top sensors 209a, 209b detect a full load state of the carrier pallet where the load of the carrier pallet exceeds the limit by reducing the weight of the load to the height thereof.
If, on the other hand, the carrier pallet 206 carries workpieces 207 whose overall height exceeds a predetermined limit, the full load state of the carrier pallet 206 is detected by the top sensors 209a, 209b.
The above described known apparatus for detecting a pallet full load state is, however, accompanied by a difficulty of maintaining a constant detection accuracy due to the fact that the total weight of the load is reduced to the overall height of the load to detect a full load state of the carrier pallet particularly when the carrier pallet carries workpieces with different sizes.
This problem will be described below in greater detail by referring to FIG. 2 of the accompanying drawings, illustrating the difference in the overall height when workpieces with different sizes are loaded to a total weight of 2 tons.
As shown, the overall height of 3'.times.6' workpieces is about 153 mm and that of 4'.times.8' workpieces is about 87 mm, whereas the overall height of 5'.times.10' workpieces is about 55 mm.
Thus, the difference in the overall height is significant for the three groups of workpieces with different sizes if the total weight is same.
Therefore, if upper limit is set to be equal to 2 tons for the total weight of the workpieces carried by a carrier pallet, the corresponding upper limit for the overall height of the workpieces will have to be equal to 55 mm which is the overall height of the 5'.times.10' workpieces weighing 2 tons.
Apparently, this upper limit for the overall height is inappropriate for 3'.times.6' or 4'.times.8' workpieces because the total weight of the workpieces of either category loaded to this upper limit height will be far below 2 tons.
In other words, the above described known apparatus for detecting a pallet full load state cannot show a constant level of detection accuracy and, as a result greatly reduces the efficiency of loading carrier pallets with workpieces.
Under these circumstances, there is a strong demand for technological development that provides a method and an apparatus for detecting a full load state that ensures a constant level of detection accuracy if workpieces with different sizes are loaded on the carrier pallet.
On the other hand, in the operation of loading carrier pallets with machined workpieces in the above described known sheet metal machining line, an additional carrier pallet is fed to the loading station only when a full load state is detected for the current carrier pallet to prolong the overall time period of a processing cycle and reduce the efficiency of the operation of the machining line.
This problem will be described in greater detail below.
The upper limit of the load of machined workpieces is determined for a carrier pallet in terms of either the total weight or the overall height of the load as a function of the height and strength of the shelf of the automatic warehouse that receives the pallet and the strength of the crane car as well as other factors including the strength of the truck for transferring the carrier pallet.
The current carrier pallet has to be replaced by an empty pallet whenever it becomes full of load.
The control apparatus of the known sheet metal machining line feeds the loading station with a replacing carrier pallet good for the machined workpieces to be loaded there only when it detects a full load state on the current carrier pallet to entail the above identified problem.
Known attempts for bypassing this problem include the arrangement of a stand-by station located adjacent to the loading station for keeping an empty carrier pallet adapted to the next unit machining schedule.
However, such an arrangement cannot cope with the situation where a full load state occurs during the ongoing current unit machining schedule and the empty carrier pallet in the stand-by station for the next unit machining schedule is not good for the current unit machining schedule.
If such is the case, the empty carrier pallet in the stand-by station has to be replaced by another empty carrier pallet that is good for the current unit machining schedules to consequently consume additional time for obtaining the empty carrier pallet.
In view of these and other circumstances, therefore, there is a strong demand for technological development that provides a method and an apparatus for anticipating a full load state for a carrier pallet and keeping an empty carrier pallet good for the machined workpieces to be loaded immediately in a stand-by station.
On the other hand, in a sheet metal machining line having a configuration as described above, the automatic warehouse plays an important role for feeding the NC machining center with necessary workpieces on a just in-time basis. The automatic warehouse typically comprises multi-column multi-level racks for temporarily storing workpieces including rough workpieces, half-finished products and finished products, a stacker-crane for moving workpieces into and out of the racks and a control apparatus for controlling the operation of the stacker-crane and other pieces of equipment in a coordinated manner. The control apparatus issues commands to the stacker-crane for moving workpieces of specified groups into and out of the automatic warehouse and centrally controls the inventory of workpieces in terms of number and column and level of rack. Therefore, when the number of workpieces of a specific group falls under a predetermined level, it notifies the operator of the current status and have the operator supply additional workpieces of the group to the automatic warehouse.
For storing newly supplied workpieces in such an automatic warehouse, the operator in charge of workpiece supply conventionally checks the invoice to find out the material, the dimensions and the weight of each piece as well as the number of pieces for each group or category of workpieces and then manually enters these data into the control apparatus typically by way of a keyboard to update the inventory data of the workpieces stored in the automatic warehouse. The updated inventory data are important for the operation of the sheet metal machining line that is being run according to a predetermined schedule because uninterrupted supply of right workpieces is vital for the performance of the line particularly in terms of the number of workpieces of each group to be fed to the line and the material, the dimensions and the weight of each workpiece of the group.
However, such conventional inventory updating operation for newly supplied workpieces that is typically carried out manually by the operator in charge of workpiece supply is cumbersome and hence he or she is mostly occupied by the operation for the day. Further, the operation is often accompanied by keying-in errors that can cause the inventory data stored in the control apparatus to be inaccurate and unreliable.
In view of the above circumstances, it has been desired to fully automate the operation of updating workpiece inventory data by providing a technological development that can realize such automated operation.